[UniverseHacker]

I'm using a desktop CO2 meter, they're cheap and accurate, and can be tested by just putting them outdoors and confirming that they read known outdoor levels correctly. What you are saying is not unlike doubting that it's possible to measure temperature with a thermometer- CO2 measurement is extremely reliable, cheap, and mature technology.

This is the meter I am using, costs $39, and is a calibrated instrument with +/- 3% accuracy. I have an academic colleague that uses these same devices for scientific research on plant metabolism, and they are highly accurate and are optionally self calibrating just by opening a window or putting them outdoors.

https://www.co2meter.com/collections/indoor-air-quality/prod...